This invention relates to a top-entry type fast reactor into which a cold leg piping and a hot leg piping are inserted from an upper portion of a reactor vessel, and more particularly to an in-vessel structure for a fast reactor, provided with a plurality of annular fins fixed to both the outer circumferential surface of an upper core structure and the inner circumferential surface of a reactor vessel, whereby the fluctuation of a liquid surface is prevented to enable the safety of an operation of a fast reactor to be secured, and a plant operation controllability to be improved.
In a demonstration fast reactor designing in which a reactor vessel is made to smaller dimensions so as to improve the economical efficiency, the flow velocity of a coolant in the reactor vessel increases greatly due to the reduction of the dimensions of the reactor vessel and an increase in the reactor output. Therefore, cover gas entrainment in the coolant due to the interaction of the high-velocity coolant with a free liquid surface is apprehended. In a comparatively small-sized experimental fast reactor, a structure in which cold and hot leg pipings for a coolant are welded to a side wall of a bottomed cylindrical reactor vessel is employed. In this structure, a dip plate suspended from an upper shielding member is employed in some cases so as to prevent the cover gas entrainment in the coolant by suppressing the rippling of a free liquid surface of the coolant.
In a large-sized fast reactor, a top-entry type (top-feed and top-discharge type) structure in which cold and hot leg pipings are inserted from an upper portion of a reactor vessel is discussed with a view to simplifying the reactor vessel structure and improving the economical efficiency. In such a top-entry type large-sized fast reactor, the dip plate suspension structure cannot be employed, since the pipings of a primary main cooling system are inserted from an upper portion of the reactor vessel, and a plurality of pipings extend between the free liquid surface and the upper shielding member. In order to prevent the gas entrainment in the coolant by optimizing an in-vessel fluid flow in the top-entry type large-sized fast reactor, a construction has been proposed in which a flow guide structure comprising a horizontal ring plate of a large width extending over the whole inner circumferential surface of a reactor vessel, and a cylindrical perforated plate extending downward from an inner edge of the ring plate is provided (refer to "Study on Flow Optimization in Reactor Vessel of Top-Entry Loop-Type DFBR", ICONE-3 (3rd International Conference on Nuclear Engineering) vol. 1, Kyoto, Japan (April 1995)).
Such a top-entry type fast reactor is shown schematically in FIG. 6. A reactor core 12 is positioned in the interior of a reactor vessel 10, and an upper opening of the reactor vessel 10 is closed with a shielding plug 14, to which an upper core structure 16 is fixed. The upper core structure 16 includes therein various instrumentation devices and supporting members of a control rod-driving mechanism (not shown). A cold leg piping 18 is introduced from an upper opening of the reactor vessel 10 into a high-pressure plenum 20, while a hot leg piping 22 is led out from an upper plenum 24 so as to pass through the upper opening. A coolant sodium is supplied from the cold leg piping 18, enters the high-pressure plenum 20, and passes through a low-pressure plenum 26 to reach the core 12, in which the coolant sodium is heated. The heated coolant sodium flows out from a core outlet surface 12a into the upper plenum 24, and further flows through the hot leg piping 22 to reach an intermediate heat exchanger (not shown) disposed outside the reactor vessel. A part of the coolant sodium passing through the low-pressure plenum 26 flows out to an intermediate plenum 28.
A flow guide structure 30 is provided in the interior of the upper plenum 24. This flow guide structure 30 comprises a horizontal ring plate 32 of a large width provided so as to extend over the whole inner circumferential surface of the reactor vessel, and a cylindrical perforated plate 34 extending downward from an inner edge of the ring plate 32. The width of the ring plate 32 is set broadly so that it becomes not less than a half of a distance between the inner circumferential surface of the reactor vessel 10 and the outer circumferential surface of the upper core structure 16. The sizes now under discussion of various parts are, for example, as follows.
Diameter of the reactor vessel: 9.88 m PA1 Diameter of the upper core structure: 2.85 m PA1 Height of the free liquid surface: 6.10 m PA1 Position of the annular plate: 2.94 m below the free liquid surface PA1 Width of the ring plate: 2.00 m PA1 Height of the cylindrical perforated plate: 1.00 m
When the flow guide structure 30 is not provided, an in-vessel flow shown in FIG. 7A occurs. When the flow velocity of the coolant sodium increases, the coolant sodium leaving the core outlet surface 12a impinges upon a lower surface of the upper core structure 16 to form a diagonal flow advancing toward the reactor vessel wall. This diagonal flow impinges upon the reactor vessel wall, and rises along the same wall. This upward flow reaches the free liquid surface 50 and swells the same. Then, the flow runs toward the upper core structure 16 along the free liquid surface 50 and runs downward along the upper core structure. Due to such a flow of the high-velocity coolant, the rippling of the free liquid surface 50 becomes violent to cause the gas entrainment in the coolant to occur. On the other hand, when the flow guide structure 30 is provided as a rippling preventing means, an in-vessel flow shown in FIG. 7B occurs, i.e., the greater part of the high-velocity coolant sodium flowing out from the core outlet surface 12a is guided to the interior of the flow guide structure 30 and blocked by the cylindrical perforated plate 34. Thus, the fluctuation of the free liquid surface during a rated operation of the reactor is eliminated.
However, when such a flow guide structure 30 comprising the ring plate 32 and cylindrical perforated plate 34 is provided, the following problems arise, though the fluctuation of the free liquid surface 50 during a rated operation of the reactor can be prevented. When the reactor is emergency-stopped (plant tripping occurs) as shown in FIG. 8, low-temperature low-velocity sodium enters directly the interior of the hot leg piping 22 in which high-temperature sodium gathers as originally designed. Therefore, there is the possibility that excessive thermal transition (cold shock) be imparted to the whole of the primary main cooling system for the reactor.
When the flow guide structure 30 comprising the ring plate and cylindrical perforated plate is provided and when a thermal stratification phenomenon occurs in the reactor vessel due to a density-difference effect of the coolant as shown in FIG. 9, a coolant flow passage is shut off by the flow guide structure 30, and the coolant in the upper plenum 24 is not agitated. As a result, the high-temperature sodium is left alone, and much time is required before the disappearance of a thermal stratification interface 60. At the thermal stratification interface 60, the portion of the reaction vessel 10 which contacts the high-temperature sodium and the portion thereof which contacts the low-temperature sodium have a thermal expansion difference, and large strain would therefore occur in the reactor vessel 10.
Furthermore, the flow guide structure 30 comprising the ring plate and cylindrical perforated plate cannot prevent the sloshing of the coolant at the free liquid surface 50 during an earthquake, and it is also difficult to prevent the fluctuation of the liquid surface accompanied by the thermal shrinkage of the coolant (liquid level change) at the time of the plant tripping.